"RST Engineering" wrote:
That's because it ain't $120 worth of jacks and wire. It is $120
worth of amplifier that takes those few millivolts of microphone
audio, uses the bias voltage coming down the aircraft radio mic audio
line, smooths that voltage into a working voltage for an amplifier,
and provides an amplifier that takes those millivolts to volts to
drive the aircraft radio.

I suspect the decimal point needs adjusting - I would have expected a
cost on the order of $12 rather than $120. Jameco carries a number of
educational audio kits that give an idea of the cost of the underlying
hardware needed to perform the task.[1] But unless one is comfortable
with electronics and one has more time than money, presumably the
simplest option is to buy the $120 system.

[1] http://www.jameco.com/webapp/wcs/sto...egoryId=703005